Inorganic deposit inhibitors, in particular in oil wells, particularly in high pressure/high temperature conditions

ABSTRACT

The invention concerns inorganic deposit inhibitors, in particular in oil wells. Said inhibitors comprise a water soluble copolymer consisting of (in mol % of polymerised monomers): a) 5 to 95% of styrenesulphonic acid or one of its salts; b) 5 to 95% of vinylsulphonic acid or one of its salts; c) 0 to 20% of one or several non-ionizable unsaturated monomers. They are particularly efficient against calcium carbonate and/or barium sulphate deposits. They are capable of being used in accordance with a so-called squeeze process in HP/MT conditions with desorption compatible with the desired treatment duration.

[0001] The present invention concerns inorganic deposit inhibitors, and particularly of calcium carbonate and barium sulphate in oil wells. It also concerns certain copolymers particularly suitable for this application, particularly in high pressure/high temperature conditions (HP/HT).

[0002] The deposition of inorganic salts, generally described as scale, is a frequent problem in operations aimed at oil recovery. One of the reasons for the precipitation of such salts resides in the mixture of incompatible fluids, containing distinct ions, which are likely to combine and precipitate, forming this scale. Thus, in offshore exploitations, large quantities of sea water containing high concentrations of sulphate ions are injected into the reservoir containing barium, calcium and possibly strontium ions. The coming into contact of these two fluids leads to the precipitation of barium, calcium and strontium sulphates both in the production well and in the surface installations and other pipe work. The calcium carbonate is formed by modification of thermodynamic equilibria linked to pressure and temperature variations.

[0003] The aforesaid observations have led to the development of anti-deposition or anti-scale additives. Thus, in patents U.S. Pat. No. 4,710,303 and U.S. Pat. No. 5,092,404, the use of sodium vinylsulphonate polymers has been suggested to inhibit the precipitation of barium sulphate in water at low pH.

[0004] In patent U.S. Pat. No. 3,879,288, the use of a copolymer of a vinylic compound has been suggested, for example (meth)acrylic acid and a vinylsulphonate or alkylsulphonate to control salts and, in particular, carbonates and sulphates of calcium and other alkaline earth metals.

[0005] In patent U.S. Pat. No. 4,898,677, inhibiting the formation of barium sulphate is suggested by adding a copolymer of acrylic acid and alkali metal vinylsulphonate, with the aim of increasing the solubility or the dispersability of barium sulphate in the medium and modifying the barium sulphate crystal structure.

[0006] In patent EP-B-297049, preventing the deposition of barium sulphate is suggested by using an appropriate quantity of a water soluble polymer containing units derived particularly from vinylsulphonic acid (10 to 100%), units derived particularly from acrylic acid (0 to 90%) and units derived particularly from unsaturated compounds with amide, acid or sulphonic ester, phosphoric, alkene, and styrene groups (0 to 25%), this patent only describing vinylsulphonic acid/acrylic acid copolymers.

[0007] Patent application EP-A-792998 suggests inhibiting in particular the formation of barium sulphate by introducing, into the aqueous fluid, a water soluble polymer comprising units derived from an unsaturated sulphonic acid and its soluble salts (5 to 35%), from an unsaturated monocarboxylic acid (0 to 85%), from an unsaturated dicarboxylic acid (0 to 80%) and from a non-ionisable unsaturated monomer (0 to 20%), the aqueous fluid containing calcium, barium, strontium and sulphate ions.

[0008] According to this document, this treatment applies in particular to the so-called “squeeze” process. This process generally consists in injecting sea water into the well, then injecting the fluid containing the inhibitor into the reservoir and injecting once again a new flow of water, with the aim of spreading out the inhibitor in the reservoir in order for it to be adsorbed on the rock.

[0009] It is interesting to note that, in the references cited above, and although calcium carbonate is cited among the salts forming scale, in fact the authors have essentially been trying to prevent the precipitation of barium sulphate under mild pressure and temperature conditions. However, it appears that certain anti-scale additives, such as terpolymers based on sulphonated monomer/acrylic acid/maleic acid, only give good results at moderate temperature (less than 150° C.). Around 20020 C., sodium vinylsulphonate homopolymers have satisfactory thermal stability but insufficient adsorption on the rock.

[0010] European patent application EP-A-459 661 relates to a process for controlling the formation of deposits of silica or silicate in an aqueous system, in which is used (a) certain water soluble, low molecular weight copolymers or terpolymers of (meth)acrylic or maleic acid, (b) a magnesium ion, (c) a mixture of said copolymers or terpolymers with an aluminium or magnesium ion, or (d) poly(meth)acrylic or polymaleic acid of low molecular weight with an aluminium or magnesium ion.

[0011] U.S. Pat. No. 4,566,972 relates to a process for preventing deposits of scale in an aqueous system, by the introduction into the water of a specific amino-carboxylate and a water soluble copolymer that may possess sulphonated units.

[0012] European patent application EP-A-184 894 relates to a process for preventing corrosion and the formation of scale and iron oxide deposits in aqueous systems, in which are introduced, into aqueous systems, a water soluble polymer prepared from an unsaturated carboxylic acid, an unsaturated sulphonic acid and an unsaturated poly(alkylene oxide).

[0013] A first aim of the invention is inorganic deposit inhibitors that can be used in high pressure/high temperature conditions.

[0014] A further aim of the invention is inorganic deposit inhibitors that can be used in the “squeeze” process.

[0015] More particularly, an aim of the invention is inhibitors with good adsorption on the rock allowing slow desorption, as water is produced.

[0016] A yet further aim of the invention is inhibitors that are particularly efficient against the deposition of calcium carbonate and/or barium sulphate.

[0017] Other aims and advantages of the inhibitors according to the invention will become clear on reading the description that follows.

[0018] The invention thus firstly relates to inorganic deposit inhibitors, characterized in that they comprise a water soluble copolymer comprising (in mole % of polymerised monomers):

[0019] a) 5 to 95% of styrenesulphonic acid or one of its salts;

[0020] b) 5 to 95% of vinylsulphonic acid or one of its salts;

[0021] c) 0 to 20% of one or several non-ionisable unsaturated monomers.

[0022] More particularly, the invention relates to inhibitors, characterized in that they comprise a water soluble copolymer comprising (in mole % of polymerised monomers):

[0023] a) 10 to 45% of styrenesulphonic acid or one of its salts;

[0024] b) 55 to 90% of vinylsulphonic acid or one of its salts.

[0025] The invention also relates to water soluble copolymers such as those mentioned in the previous paragraphs.

[0026] As indicated previously, the aforesaid sulphonic or polycarboxylic acids may be replaced partially or totally by their salts, particularly alkali metal salts such as sodium or potassium or ammonium or quaternised amine salts.

[0027] Among the styrenesulphonic acids (and their salts), 4-styrenesulphonic acid is preferably used. When the copolymer comprises units derived from unsaturated polycarboxylic acids (or their salts), maleic acid or its anhydride, fumaric, itaconic, citraconic acids or cis 1, 2, 3, 6 tetra-hydrophthalic anhydride or water soluble sodium, potassium or ammonium salts of said acids are preferably used. Such monomers, when they are present, represent preferably up to 30% in mole % of the polymerised monomers.

[0028] Among the non-ionisable unsaturated monomers, non-substituted or (C1-C8) alkyl or hydroxy (C1-C8) alkyl substituted (meth)acrylamides; non-substituted or (C1-C8) alkyl- or hydroxy alkyl substituted (meth)acrylic esters; vinyl acetate, styrene, vinyltoluene, may, in particular, be cited. Such monomers, when they are present, represent up to 10% in mole % of the polymerised monomers.

[0029] When the copolymer only comprises units derived from derivatives of vinylsulphonic acid or its salts and styrenesulphonic acid and, in particular, 4-styrenesulphonic acid or its salts, the molar percent of vinylsulphonic acid or its salts is preferably between 70 and 90% and, even more preferably, between 80 and 90%.

[0030] The invention especially relates to such water soluble copolymers.

[0031] The water soluble copolymers according to the invention have a molar mass (by number) generally between 3 000 and 50 000 and preferably between 5 000 and 30 000. Preferably, their dispersity index M_(w)/M_(n) is less than or equal to 3.

[0032] The molar mass of copolymers is measured by steric exclusion chromatography (SEC). The chromatographic chain used comprises a WATERS 590 pump (WATERS, Milford, USA), a WATERS 717plus injector and a WATERS R410 refractomeric detector. The set of columns used comprises 2 columns (POLYMER LABORATORIES PL aquagel-OH; 8 μm; 300×7.5 mm) filled with rigid, macroporous polymer beads (very hydrophilic, polyhydroxylated surface). Two porosities are used: 40 (resolution field: 10000-200000 eq. PEO/PEG) and 30 (resolution field: 100-30000 eq. PEO/PEG). The eluant is composed of 80% water (purified with MILLI-Q ZFMQ 230 04 from MILLIPORE) containing sodium nitrate at a concentration of 0.3M, sodium hydrogen phosphate at a concentration of 0.01M and 20% methanol; the solution thus has a pH of 9. The rate at which the solvent is introduced is 1 cm³.min⁻¹. The copolymer solutions to analyse are prepared at a concentration of 1 g.l⁻¹ and 100 μl of solution are injected. The calibration curve is constructed from a set of 8 PULLULAN polysaccharide standards with M_(w) between 5900 and 788000, with a narrow distribution (dispersity index between 1.06 and 1.23).

[0033] The distribution of molar masses is calculated by using the results of refractomeric detection and the calibration obtained from the PULLULAN standards. Finally, different characteristic average sizes for the distribution are calculated (Mn: average mass by number; Mw average mass by weight; I: dispersity index=Mw/Mn).

[0034] The copolymers according to the invention may be obtained by polymerising the monomers mentioned previously using conventional methods in the presence of free radical initiators.

[0035] Depending on the solubility of the monomers used, the polymerisation is carried out in aqueous solution or partially aqueous solution, for example a water/alkanol solution, or an alkanol such as ethanol, isopropanol, ethylene glycol, diethylene glycol.

[0036] The reaction medium is advantageously at acid pH, preferably less than 6.

[0037] Among the free radical initiators, one may cite peroxides, such as benzoyl or t-butyl peroxides, azo compounds, such as azo-bis-isobutyronitrile, 2, 2′-azo-bis(2-amidino-propane)dihydrochloride (AIBA) and, preferably, peracid salts, such as potassium or ammonium persulphates.

[0038] The polymerisation is generally carried out at a temperature between 40 and 120° C., preferably between 45 and 100° C., and the polymerisation time may be several hours, for example 2 to 10 hours.

[0039] The copolymers are obtained in solution form and this may be subjected to partial or total evaporation, generally under reduced pressure. The copolymers that have undergone partial or total evaporation are re-diluted before their use.

[0040] The copolymers in solution may be used as such as inhibitors according to the invention, or purified in accordance with conventional methods such as:

[0041] evaporation of the reaction solvent, dissolution in water, washing with a non water miscible organic solvent (ether) and evaporation of the aqueous solution, or

[0042] evaporation of the reaction solvent, dissolution of the residue in methanol and precipitation by the addition of ether.

[0043] The copolymers according to the invention, in which all or part of the acid hydrogens are replaced by cations derived from an appropriate salt forming a base may also be prepared by mixing an aqueous, water/alcohol or alcohol solution of the acid copolymer with an aqueous, water/alcohol or alcohol solution containing a quantity of the required base appropriate to the desired level of substitution.

[0044] The copolymers according to the present invention constitute inorganic deposit inhibitors, particularly of calcium carbonate and barium sulphate, and are particularly efficient when used in the “squeeze” process in oil wells operating under high pressure conditions, in other words around 20 to 150 MPa and high temperature, in other words around 130 to 230°C. These copolymers have in fact sufficient thermal stability, have good efficiency in inhibiting the formation of salts (CaCO₃ and BaSO₄), are insensitive to high levels of Ca⁺⁺ of the reservoir water and have sufficient adsorption on the rock for use in “squeeze” processes. This set of properties is not found in inhibitors known to the prior art, whether they are terpolymers based on sulphonated monomer/acrylic acid/maleic acid or homopolymers of sodium vinyl sulphonate.

[0045] Without being tied to a particular explanation, one could suppose that these properties are due to the presence of styrenesulphonate groups in the copolymers: these groups provide insensitivity to the calcium of copolymers of high molecular weight compared to homopolymers with the same molecular weight, and thus greater adsorption on rock.

[0046] During transportation and storage operations, the inhibitors according to the present invention are either in powder form, or in solution whose concentration may be around 15 to 30% by weight.

[0047] When used in oil wells, the solutions intended for continuous injection in the wells have a concentration of around 1 to 100 ppm, whereas the solutions used in “squeeze” processes are injected so that they are placed on the rock at a concentration of around 10% by weight, all of these concentration values being only given by way of indication.

[0048] Although the inhibitor according to the invention may be used alone, one may, without going beyond the scope of this invention, combine it with additives known in the application field, such as corrosion inhibitors, dispersing agents, precipitation agents, oxygen scavengers, sequestering agents, anti-foaming agents, biocides, de-emulsification agents, anti-asphaltenes.

[0049] The application properties of the inhibitors according to the invention are evaluated as follows:

[0050] a) Inhibition of BaSO₄ and CaCO₃

[0051] This test enables the efficiency of the deposit inhibitors used in preventive treatments to be evaluated. Performed on an anti-deposition loop, it consists in measuring the clogging time of a capillary when a mixture of two incompatible waters is injected: one water contains bicarbonates (HCO₃ ⁻), the other calcium (Ca⁺⁺ Mg⁺⁺) for a study of the carbonated deposits (calcite, etc.); one water contains sulphates (SO₄ ⁻), the other barium (Ba⁺⁺, Sr⁺⁺) for a study of the sulphated deposits. Each of the waters contains a quantity of inhibitor that may generally vary between 10 and 100 ppm. Measuring the differential pressure enables the evolution of the clogging of the capillary to be monitored. The experiments comprise a control test without inhibitor and tests in the presence of inhibitors.

[0052] b) Compatibility with Calcium

[0053] One determines from what concentration in Ca⁺⁺ an inhibitor, at a given concentration, is going to precipitate. This is observed by monitoring the evolution of the cloudiness of the contact solution using a turbidimeter.

[0054] The principle of the test is to observe the appearance of cloudiness or a deposit by the decrease in the transmittance of distilled water solutions of each inhibitor (at 0.1, 1.5 and 10% by weight at pH 5, 6 and 7 when known quantities of calcium are added. The tests are carried out at ambient temperature.

[0055] c) Rock Adsorption Tests (Core Test)

[0056] This test is used for dimensioning the preventive treatments by the “squeeze” process. It is carried out on a representative core sample from the well in which the treatment will be carried out, and its purpose is to verify that the adsorption (or the precipitation) of the inhibitor on the porous medium is correct and thus that the treatment will have an acceptable lifetime and to verify that the adsorption (or the precipitation) of an important quantity of inhibitor will have no effect on the well productivity (reduction in the permeability). The experimental set up employs a “HASSLER” cell in which the core sample is characterised (porosity, etc.) and conditioned to the well conditions (pressure, temperature, presence of oil). The different sequences of a squeeze treatment (with the exception of overflush) are reproduced: preflush, flush, closing, formation water flush. A fraction collector at the output of the system enables samples of fractions to be collected during the injection and flushing phases in order to monitor changes in concentrations of inhibitor (adsorption/precipitation, desorption). At the end of this test, the experimental data is used to calculate an adsorption (or precipitation) law for the inhibitor, which will then be used to dimension the on-site treatment.

[0057] The examples that follow illustrate the invention.

EXAMPLES 1 to 5 AND COMPARATIVE EXAMPLES 1 a to 1 f 1. Copolymer Synthesis

[0058] Example for continuous addition (test 3).

[0059] The quantities of reagents and additives and the results are shown in Table I, where VS and SStNa designate respectively sodium vinylsulphonate and sodium 4-styrenesulphonate.

[0060] The sodium 4-styrenesulphonate is dissolved in water and, in a proportion of around 10% by weight of this solution, the sodium vinylsulphonate solution is added. The pH is checked and sulphuric acid (96% by weight) is added if necessary in order to obtain a pH of 3.8.

[0061] This solution of monomers is introduced into a stainless steel reaction vessel equipped with a double agitator. The vessel is degassed with nitrogen 3 times in a succession of vacuum/nitrogen cycles.

[0062] The solution is then heated under agitation (250 rpm). Simultaneously, potassium persulphate in aqueous solution is added in a single operation or continuously at a flow rate of 25 cm³/h (see table) and the remainder of the solution of SStNa added continuously at the same speed.

[0063] The temperature is maintained for 1 to 2 hours, or the polymerisation is carried out at two different temperatures (see Table I).

[0064] It is then allowed to cool to 40° C. and the solution removed.

[0065] A polymer solution at a concentration of 20% by weight is obtained.

EXAMPLE FOR BATCH (TESTS 1, 2, 4, 5, 1a and 2a)

[0066] The SStNa is dispersed and/or dissolved in the VS solution. The pH is adjusted to 3.8. It is then heated to the reaction temperature. The initiator is added and the solution left for 2 to 10 hours.

[0067] 2. - Application Properties

[0068] The inhibitors are used in the form of aqueous solutions at different concentrations according to the test and are subjected to the tests described above.

[0069] The results are shown in Table II.

[0070] In the “Additive” column, the symbols have the following meaning:

[0071] VS=sodium vinylsulphonate (monomer)

[0072] HVS=sodium vinylsulphonate homopolymer

[0073] VS/VSS=sodium vinylsulphonate/sodium styrenesulphonate copolymer, with the molar proportions of the monomers polymerised given.

[0074] TROS810=HVS commercialised by the TROS Company

[0075] EC6151A=HVS commercialised by the Nalco/Exxon Company.

[0076] In the “thermal stability” column, the symbol OK signifies a stability verified after 3 weeks at 200° C.

[0077] In the “calcium compatibility” column, the term “compatible” signifies the absence of cloudiness or deposit, for a quantity of calcium up to 40 g/l and a pH of the medium from 4 to 7.

[0078] In the “adsorption” column, the quantity of inhibitor adsorbed per gram of rock is shown (in mg): the rock used had the following characteristics:

[0079] 209 g core sample with a pore volume of 16.5 cm³.

[0080] Sandstone

[0081] An aqueous solution with 10% inhibitor was injected while maintaining the rock at a temperature of 200° C. (use in “squeeze”).

[0082] In the “desorption” column, the lifetime of the treatment is indicated (in days) at the end of which the concentration in inhibitor in the production water was respectively 1, 5, 10 or 50 ppm. The values were obtained by simulation using a “Squeeze 5” software, developed by Heriott Watt University in Edinburgh. This simulation was carried out using the following operating conditions:

[0083] deposit thickness: 30 m

[0084] injection of 220 m³ of solution with 10% inhibitor

[0085] production of water: 500 m³/day and by integrating the adsorption values from Table II. TABLE I Polymerisation conditions Monomers (g) Temperature Copolymer obtained VS Water Operating (° C.) and Initiator Molar mass Composition (moles) Example (at 25%) SStNa H₂SO₄ (g) (g) conditions duration (g) Mn/Mw VS VSS 1  187.3 8.25 0 20 batch 50/6 h K₂S₂O₈ 10670 90 10 2.16 18190 2  176.9 12.4 0 50 batch 50/6 h K₂S₂O₈ 7745 85 15 2.16 22030 3  442.3 3.09 0.9 250 Continuous 80/2 h K₂S₂O₈ 5530 85 15 addition of + 5.4  12350 initiator over 2 90/1 h h + remainder 27.81 of SstNa solution 1a 416.3 0 0 100 batch 50/4 h K₂S₂O₈ 7607 100 — 4.32 9141 2a 416.3 0 0 200 batch 50/4 h K₂S₂O₈ 8292 100 — 1.1  10440 4  468.4 22.9 1.8 150 batch 50/6 h K₂S₂O₈ 12500 90 10 5.4  17670 MBS 3.8  5  468.4 22.9 1.2 25 batch 70/6 h AIBA 8321 90 10 5.4  21940

[0086] TABLE II Efficiency test on deposits Desorption in days Clogging time in hours (simulation) Additive Molar mass Thermal BaSO₄ ⁻50 Calcium Adsorption on 1 5 10 50 Example tested Mn/Mw stability ppm CaCO₃ 10 ppm compatibility the rock (mg/g) ppm ppm ppm ppm 1e None — — 0.8 1.3 1f VS monomer — NO 0.4 2.1 Compatible 1  VS/VSS 10670 OK 6.5 3.1 Compatible 1.75 390 286 209 37 90-10 18190 2  VS/VSS 7745 OK 7.0 5.3 Compatible 85-15 22030 3  VS/VSS 5530 OK 6.5 5.0 Compatible 85-15 12350 1a VS 7607 OK 6.8 3.3 Compatible 100 9141 1b VS 8292 6.8 6.2 Compatible 100 10440 1c TROS 810 13270 OK 6.9 3.9 Very cloudy 17820 if concen- tration > 1% 1d EC 6151A 10480 OK 7.0 5.8 Cloudy if 1 297 127 58 20 13360 pH > 7 4  VS/SS 12500 OK 6.9 — Compatible 90/10 17670 5  VS/SS 8321 OK — — Compatible 90/10 21940 

1- water soluble copolymer comprising (in mole % of polymerised monomers): a) 5 to 95% of styrenesulphonic acid or one of its salts; b) 5 to 95% of vinylsulphonic acid or one of its salts; c) 0 to 20% of one or several non-ionisable unsaturated monomers, with the exception of monomers of the following formula:

wherein R¹ and R² are, independently of each other, a hydrogen, a C₁ to C₄ alkyl, an ammonium ion or an alkali metal or alkaline earth metal ion. 2- Copolymer according to claim 1, consisting of (in mole % of polymerised monomers): a) 10 to 45% of styrenesulphonic acid or one of its salts; b) 55 to 90% of vinylsulphonic acid or one of its salts. 3- Copolymer according to claims 1 or 2, wherein all or part of the sulphonic acids are replaced by their alkaline metal salts or their ammonium salts or their quaternised amine salts. 4- Copolymer according to any of claims 1 to 3, wherein the styrenesulphonic acid is 4-styrenesulphonic acid. 5- Copolymer according to any of claims 1 to 4, with a molar mass (by number) between 3 000 and 50 000 and preferably between 5 000 and 30
 000. 6- Copolymer according to any of claims 1 to 5, with a dispersity index M_(w)/M_(n) less than or equal to
 3. 7- Inorganic deposit inhibitor consisting of a copolymer according to any of claims 1 to
 6. 8- Process for preparing a copolymer according to any of claims 1 to 6, wherein it comprises polymerising monomers in aqueous, water/alcohol or alcohol solution, in the presence of a free radical initiator and at a temperature of between 40 and 120° C. 9- Use of the inorganic deposit inhibitor according to claim 7 in oil wells.
 10. Use according to claim 9 according to the squeeze process, consisting in injecting sea water into the well, then injecting the fluid containing the inhibitor into the well and injecting once again a flow of sea water. 